1. Field of the Invention
The present invention relates to a semiconductor device in which a thin metal wire made of copper or a copper alloy is formed at an electrical connection portion.
2. Description of the Related Art
Manufacturing steps of a semiconductor device are roughly classified into a pre-processing step of forming functional elements such as circuits or devices on a semiconductor substrate and an assembly step of assembling the semiconductor substrate on a lead frame.
In this assembly step, a method of utilizing a lead frame is mainly used regardless of whether an IC or an individual semiconductor element is to be manufactured.
In the manufacture of a semiconductor element in which a packing density is significantly increased as a massproduction technique progresses, various steps are automatically performed to eliminate a manual operation which easily produces waste materials. In addition, a multifunctional machine obtained by combining manufacturing apparatuses required in a plurality of steps is used in an assembly step and the like.
This assembly step includes a variety of steps. Of these steps, bonding, i.e., a thermocompression bonding technique concerned with the present invention and a resin encapsulating technique performed subsequently to the bonding will be briefly described below with reference to FIG. 1.
Although a variety of types of lead frames are used, in the case of a multipin type lead frame such as an IC or the like, a plurality of substantially rectangular hollow metal frame members 60 are continuously formed, and a plurality of lead terminals 61 are formed such that each lead terminal is formed in a centripetal direction from each frame member as a unit member, as shown in FIG. 2A. As shown in FIG. 1, a large-diameter bed portion 50 is formed at one end of each lead terminal (not shown), and a semiconductor substrate 51 on which a semiconductor element is formed by a conventional method is mounted by a known mounter.
In the semiconductor element formed on the semiconductor substrate 51, a thin metal wire 53 is formed between an electrode 52 formed adjacent to an impurity region constituting a functional element and the other lead terminal to obtain an electrical connection. For this purpose, a so-called bonding method, i.e., a thermocompression bonding method is utilized.
As the thermocompression bonding method, an ultrasonic thermocompression bonding method is usually applied, and a thin gold wire is generally used. From an economic viewpoint, however, a thin aluminum wire is also adopted in accordance with the type of device. In addition, a thin wire made of copper or a copper alloy has been developed on the basis of the fact that it has a bondability with aluminum, an aluminum alloy, copper or a copper alloy as a material of an electrode formed on a semiconductor substrate.
In the field of semiconductor elements in which cost competition is severe, products which are inexpensive and yet have excellent characteristics are desired. For this reason, a demand has arisen for the use of copper or a copper alloy in place of an expensive thin gold wire.
As described above, in the assembly step utilizing the lead frame, a thin metal wire 53 having a loop with a predetermined height is formed by the thermocompression bonding method utilizing an ultrasonic wave in consideration of a flow of a melted resin in the resin encapsulating step to be performed subsequently to the assembly step.
Examples of an electrode material to be subjected to the above thermocompression bonding step are pure Al, Al-Si, Al-Cu/Al-Si-Cu, and Al-Si-Cu. A hardness of the material, however, is not standardized although it changes in accordance with the type of metal and its formation conditions.
One method of forming the thin metal wire 53 having a loop by the ultrasonic thermocompression bonding is so-called on-element bonding. In this method, a thin metal wire is ultrasonically thermocompression-bonded to an electrode formed adjacent to an impurity region formed to constitute a so-called functional element. This method, however, is not often used because a distortion or cracks are produced in the electrode very frequently.
In a method which is generally used, as shown in FIG. 1, an opening is formed in an insulating material layer 54 coated on the surface of the semiconductor substrate 51, a wiring layer 55 connected to an electrode 52 formed in the opening is formed to extend to the surface of the insulating material layer near the end portion of the semiconductor substrate to serve as a terminal end, and a so-called bonding pad is formed thereat. In this method, therefore, the insulating material layer 54 must function as a buffer layer upon thermocompression bonding. In addition, this thermocompression bonding is so-called first bonding different from stitch bonding.
As described above, as a thin metal wire used in the thermocompression bonding method utilizing an ultrasonic wave, a material which mainly consists of copper or a copper alloy and has been used on an experimental basis is being put to practical use. A thin wire made of copper or a copper alloy, however, has a higher hardness than a thin gold wire. Therefore, a load and an ultrasonic output applied to flatten a ball 57 so as to form flattened-ball shape 571, as shown in FIGS. 3 and 4 for obtaining a proper bonding strength must be increased upon thermocompression bonding of such a thin wire.
For this reason, this load is applied on the insulating material layer formed adjacent to the electrode to which the thin metal wire is to be thermocompression-bonded and produces a distortion or microcracks very frequently, thereby degrading the reliability of a semiconductor element or device.
In order to solve the above problem, methods of
(a) increasing the thickness of the metal electrode,
(b) increasing the hardness of the metal electrode and
(c) increasing the purity of the thin metal wire have been proposed.
None of the above methods is perfect, however, and therefore the thin metal wire must be thermocompression-bonded under the conditions (a load, an ultrasonic output and the like) narrower than those for ultrasonic thermocompression-bonding a thin gold wire.